This invention relates to an effective dry etching process of actinide oxides and their mixed oxides in CF4/O2/N2 PLASMA.
The fluorination of uranium dioxide has been extensively studied in the application-oriented fields such as uranium separation, processing, and conversion. Along with the applied research, fundamental studies of the UO2/F2 reaction have been reported by several authors [T. Yahata and M. Iwasaki, J. Inorg. Nucl. Chem. 26 (1964) 1863, G. Vandenbussche, CEA-R 2859 (1966), M. Iwasaki, J. Nucl. Mater., 25 (1968) 216, J. C. Batty and R. E. Stickney, J. Chem. Phys., 51 (1969) 4475, B. Weber and A. Cassuto, Surf. Sci., 39 (1973) 83, A. J. Machiels and D. R. Olander, High Temp. Sci., 9 (1977) 3].
The reaction of UO2 at low temperatures of below 800 K under atmospheric pressure of F2 was studied using weight loss measurements by Vandenbussche [G. Vandenbussche, CEA-R 2859 (1966)] and by Iwasaki [M. Iwasaki, J. Nucl. Mater., 25 (1968) 216]. Under these conditions the ultimate reaction products are found to be UF6 and O2, while a variety of intermediate reaction products such as (UO2)4F and UO2F2 are identified. On the contrary, a quasi-equilibrium reaction modelling study predicted that at high temperatures of above 1000K under low pressure of F2 (10xe2x88x927xcx9c10xe2x88x924 Torr), uranium hexa- and penta-fluoride formations are suppressed in favor of UF4 and atomic fluorine formation [J. C. Batty and R. E. Stickney, J. Chem. Phys., 51 (1969) 447, and B. Weber and A. Cassuto, Surf. Sci., 39 (1973) 83].
A kinetic study carried out later at high temperatures of above 1,000 K under ultra-high vacuum condition confirmed that the reaction product is UF4 and the reaction probability is about 10xe2x88x922 [A. J. Machiels and D. R. Olander, High Temp. Sci,. 9 (1977) 3]. The authors claimed that the reaction mechanism is a second-order surface reaction coupled with the double-diffusion process. The disagreement between these early experimental results seems to stem from the different ranges of temperatures and pressures.
Recently the feasibility of burning spent PWR fuel in a CANDU reactor was carried out, in which decladding of spent fuel pins and dry-processing of burned uranium dioxide such as OREOX (Oxidation and Reduction of Oxide fuel) process are the main processes to make re-sinterable fuel powder [H. Keil. P. Boczar. and H. S. Park, Proc. Intern. Conf. Tech. Expo. on Future Nuclear Systems, Global ""93. Seattle, Wash., USA [Sep. 12-17, 1993) 733 and M. S. Yang. Y. W. Lee. K. K. Bae. and S. H. Na. Proc. Intern. Conf. Tech. Expo., on Future Nuclear Systems, Global ""93. Seattle, Wash. USA (Sep. 12-17, 1993) 740]. In the process, however, most candidate decladding techniques were unable to recover more than 98 to 99.5% of the heavy metal/metal oxide. A part of the remainder will be present as adherent dust and some may also be chemically bonded to the zirconium oxide layer on the inside of the fuel pin. Therefore, another process for additional removal of the last portion of the fuel is required, which also removes alpha contamination from the clad to a level qualifying the fuel hulls as non-TRU. For the secondary decontamination process, a plasma processing technique using fluorine-containing gas plasma was proposed and its applicability has been demonstrated [Y. Kim. J. Min, K. Bae, M. Yang, J. Lee. and H. Park, Proc. Intern. Conf. on Future Nuclear Systems, Global ""97. Yokohama, Japan (Oct. 5-10, 1997) 1148]. Since then, dry etching treatments of TRU oxide including uranium dioxide have been extensively focused.
Following the demonstration, as a representative compound of actinide oxides including TRU dioxides, the effective etching reaction process of uranium dioxide in CF4/O2/N2 plasma and reaction mechanisms have been investigated in detail in this work.
It is invented that the fluorination etching reaction of actinide oxides such as UO2, ThO2, and PuO2 in CF4/O2 gas plasma is enhanced when small amount of N2 gas is added or mixed at the temperature of ambience up to 600xc2x0 C. under the low pressure of 1 m Torr up to 1 atm. As a representing actinide, oxide uranium dioxide was chosen and its reaction rates were investigated as functions of CF4/O2/N2 ratio, plasma power, substrate temperature, and exposure time to the plasma. From the current investigation, it is found that there exists an optimum CF4/O2 ratio for the effective etching in CF4/O2/N2 plasma. The ratio of CF4 to O2 is around four, regardless of plasma power, substrate temperature, and gas volume flow rate. When the small amount of N2 gas ranging from 1% to 20% of CF4 gas based on the gas volume is added to or mixed with the optimized CF4/O2 the etching rate is enhanced remarkably over 4 up to 5 times compared to that of CF4/O2 plasma without N2 gas.
This optimum etching process must be applicable to the dry etching of other actinide oxides including TRU (TRans-Uranium) oxides and their mixed oxides since all actinide elements have very similar chemical characteristics with uranium and, thus, form similar types of oxides.
In current examination, r.f. and microwave power gas plasma generation techniques were used with the power ranging 50 W up to 2 kW and the effectiveness of this process was confirmed. Since basic principle of gas plasma generation techniques is identical except different working pressure ranges, this effective etching rate must be increasing with increasing plasma power up to 100 kW extractable from various gas plasma generation techniques such as dc (direct current), ac (alternating current), and ecr (electron cyclotron resonance) plasma.
Also the effectiveness of this process was successfully demonstrated in the etching experiments of uranium oxide on the zirconium alloys, stainless steels, or inconels (Ni based alloys) substrates.